The Summoned Mage — Melissa McShane

Sesskia, talented thief and secret mage, has spent the last ten years traveling the world, searching for ancient books that will teach her magic forbidden in her country. When she is magically transported to another world where magic is not only permitted, but celebrated, she wonders if she ever wants to return. With new friends and a chance at a new life, Sesskia feels as if she's finally come home.

But catastrophe is coming. Her new home Castavir and the world of her birth are coming inexorably together, heralding near-total destruction for both.

The mad God-Empress of Castavir intends to take advantage of the crisis by conquering both worlds -- whatever is left of them. Sesskia must work with the Castaviran mages to combine their radically different magics to prevent the convergence from happening and save both her worlds.